cmd/go/internal/modload: add missing build constraint in testgo.go

That file is supposed to make unexpected dependencies on the main
module easier to diagnose in 'go test cmd/go', but I accidentally left
off the build constraint, so it was triggering outside of the test.

Updates #29097

Change-Id: I1cde3fe6c1d80add37c98a8c95ce48524ea05024
Reviewed-on: https://go-review.googlesource.com/c/153159
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modload/testgo.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//+build testgo
+
 package modload
 
 func init() {